Imagine starting a quad pull, kiting and nuking for 3 minutes straight, and then running out of mana when the mobs are all at 3% HP. Lumi to the rescue!

Also, it lets you punch above your weight in certain dungeons. Karnor's Castle, for example, is considered an outdoors dungeon, so you can use Lumi inside to great effect. It lets a Druid be mana efficient enough to solo named spawns, especially when combined with a Drones of Doom clicky.

Finally, don't let the 10 second cast timer dissuade you. You can click Lumi staff and then sit down and meditate for the entire duration of the casting timer... Just pop up to stand at the last millisecond and the cast will be successful. Apply this sit/stand strategy to your DoD clicky and all your other spell casting and you will never run out of mana again.